1. Field of the Invention
The present invention is directed to a charge control circuit for a battery charger, and more particularly to a battery control circuit adapted for a battery charger to selectively providing first and second control signal for changing the rate of charge depending upon the temperature gradient of the battery being charged.
2. Description of the Prior Art
Battery chargers for storage batteries such as nickel-cadmium cells are required to be controlled by a suitable control circuit for achieving a rapid and safe charge. Basically, there have been proposed two control techniques to monitor the terminal characteristics of the battery for terminating the rapid charge before the battery is overcharged, one is to monitor the terminal voltage and the other is to monitor the battery temperature, both increasing as the battery is charged. It has also been found, as taught in U.S. Pat. No. 3,337,026, that the battery temperature is well indication of the charge state of the battery and therefore is a more consistent measure than the terminal voltage. Improvements in controlling the battery charger based upon the battery temperature measurement are proposed in U.S. Pat. Nos. 3,599,071 and 3,917,990 in which the ambient temperature is monitored together with the absolute battery temperature so as to provide a control signal as a function of the difference between the battery and ambient temperatures in an attempt to effectively charge the battery in a relatively hot or cold environment at which conditions the battery would be less- or over-charged with the control circuit only monitoring the absolute battery temperature since the initial temperature of the battery in that environment is correspondingly high or low and therefore requires less or greater time before reaching the absolute critical temperature to terminate the rapid charge.
These prior control circuits monitoring the difference between the battery and ambient temperatures, however, still have a problem that a fixed temperature difference determining the point at which the high rate charge is terminated may not always apply to different batteries which are always required not to be less- or over-charged. In other words, different settings as to the temperature difference may be required for different batteries in order to successfully charge the batteries to its full capacity, yet prevent overcharge. This is a rather complicated and inconvenient process for the charge control circuit to perform. Further, even if monitering the temperature difference works well for the different batteries there still remains a problem that the measurements of the ambient temperature may considerably fluctuate from spot to spot in actual use, thus making it difficult to determine an exact temperature difference applicable to varying environments and therefore possibly failing to provide a consistent index ensuring effective charge performance. These problems are inevitable insofar as the battery-to-ambient temperature difference is utilized as an index to terminate the high rate charge. Therefore, it is highly desirable to find an alternative parameter which is well indicative of the actual charge state of the battery without suffering from the above uncertainty for ensuring a consistent charge control over varying ambient temperature conditions.